Step 5: Depending on the technology you chose, you need to choose file formats, image sizes and of course other scope issues, make sure those file formats are supported by the technology you chose. Other scope issues like how many levels, weapons, types of enemies, types of characters, different screens. This will help keep things structured, and give you a good idea when your done.

Step 6: Design a 1st level this must be complete all the way through no gaps. Don't hold yourself back because you think something can't work, just try it and change it later if you find it doesn't work.
Step 7: If you are programming it yourself or even if you are doing a MOD or using a game making application, work on making the core gameplay mechanics first such as shooting enemies and gaining points. Getting and completing quests and leveling up etc.

Step 8: Once you have a playable 1st level with place-holder images and models and other content. It's time to finish it up, with placeholders of course, just use place holders everywhere don't worry about it looking horrible you can always make it prettier.

Step 9: Now you can make it pretty, smooth out the graphics a little, do some performance enhancements, etc... Make the art style consistent add in more effects, to make players give the "cool" and "I like it". This would be known as polishing. Remember it's only that first level.

Step 10: Depending on which technology you chose you want a Level Editor unless all your content is randomly generated in code in which case this list may not do anything for you. This is the step where you churn out content after content, basically the part of the game making you really wanted to do.